This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-291901, filed Sep. 26, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a feedforward type linearizer. The linearizer is an apparatus (circuit) that improve linearity of an amplifier. The present invention relates to a linearizer classified as a feedforward type.
2. Description of the Related Art
A feedforward type linearizer generates a signal whose phase is reverse to that of a distortion component included in an output signal of an amplification element and whose strength is equal to that of the distortion component, combines the signal with the output signal of the amplification element, and canceling the distortion component.
It is necessary to detect the distortion component for controlling this type of a linearizer with precision. However, a frequency of the distortion component fluctuates with the frequency of a carrier signal to be amplified. It is therefore difficult to steadily detect the distortion component. To solve the problem, this type of linearizer is constituted to mix a pseudo distortion component with the carrier signal before being inputted to the amplification element, detect the pseudo distortion component from the output signal of the amplification element, and thereby detect an inherent distortion component.
There is a feedforward type linearizer which uses a pilot signal as the pseudo distortion component. However, when the carrier signal belongs to a high frequency band, the necessity of generating a high-frequency pilot signal occurs. A pilot signal generator for a high frequency band has a complicated constitution and is expensive. Therefore, the use of the generator is avoided wherever possible.
An invention in which the distortion of an amplification signal can be compensated for without using a signal generator for the high frequency band is disclosed in Jpn. Pat. Appln. No. 6-066834 (filed in Japan). A distortion compensator disclosed in the publication modulates the carrier signal to be inputted to the amplification element with a modulated signal, generates a side band signal, and uses the side band signal as the pseudo distortion component. A signal belonging to a base band can be used as the modulated signal. Therefore, it is possible to generate the pseudo distortion component without using the signal generator for the high frequency band.
When the carrier signal is modulated with the modulated signal, an upper side band signal is generated on a high-frequency side of a band of the carrier signal, and a lower side band signal is generated on a low-frequency side. When the band of the carrier signal is narrow, either the upper side band signal or the lower side band signal is detected, and the distortion component of the carrier signal can effectively be suppressed.
Additionally, with the spread of modulation systems such as code division multiple access (CDMA), there has been a tendency to broaden the band of the signal inputted as the carrier signal, in recent years. In order to effectively suppress the distortion component of a broad band signal, both the upper side band signal and the lower side band signal need to be detected.
A conventional feedforward type linearizer comprises a band-pass filter for passing the upper side band signal, and a band-pass filter for passing the lower side band signal. Moreover, each filter is connected to a side band detector, so that the upper side band signal and lower side band signal are individually detected.
However, in this constitution, two systems are necessary for each of the band-pass filter and side band detector. Therefore, there is a disadvantage that the size and cost of the apparatus is increased.
To eliminate the disadvantage, there is a feedforward type linearizer for changing the frequency of the local signal. According to the constitution, one band-pass filter and one side band detector may be disposed.
However, in this constitution, a predetermined time is required until the frequency of the changed local signal is stabilized. Concretely, a lockup time of a local oscillator for generating the modulated signal, and a time for supplying a frequency changeover control signal are necessary. Therefore, the time required for detecting one side band is lengthened, which is disadvantageous as this makes it difficult to control the amplification properties at high speed.
In a CDMA communication system, when the number of multiple CDMA signals changes, the level of the transmission signal changes at high speed. Therefore, the amplification properties of the feedforward type linearizer applied to a transmitter of the CDMA signal need to be controllable at high speed.
An object of the present invention is to provide a feedforward type linearizer whose amplification property can be controlled at a high speed without increasing a size and cost.
According to the present invention, there is provided a constitution comprising: a first power distributor for distributing an input signal to a main signal and an auxiliary signal; an orthogonal modulator for orthogonally modulating the main signal distributed by the first power distributor in response to a predetermined base band signal in order to generate a side band signal; a first vector adjuster for adjusting an amplitude and phase of the main signal modulated by the modulator; a first amplifier for amplifying the main signal; a second power distributor for distributing the main signal amplified by the first amplifier; a first power combiner for combining one signal distributed by the second power distributor and the auxiliary signal distributed by the first power distributor; a main signal detector for detecting the main signal from the signal whose power is combined by the first power combiner; a second vector adjuster for adjusting the amplitude and phase of the signal combined by the first power combiner; a second amplifier for amplifying the signal sent from the second vector adjuster; a second power combiner for combining the other signal distributed by the second power distributor and the signal amplified by the second amplifier; a side band signal detector for receiving an output signal from the second power combiner, and detecting the side band signal included in the output signal; a first controller, disposed between the main signal detector and the first vector adjuster, for controlling the first vector adjuster so that a value of the main signal detected by the main signal detector is minimized; and a second controller, disposed between the side band signal detector and the second vector adjuster, for controlling the second vector adjuster so that the value of the side band signal detected by the side band signal detector is minimized.
The orthogonal modulator comprises phase reverse means for generating I and Q signals for use in orthogonal modulation from the base band signal, and mutually reversing phases of the I and Q signals in time division.
The side band signal detector comprises: a local oscillator for generating a local signal of a frequency corresponding to a middle of a band of the input signal; a mixer for mixing the local signal from the local oscillator with the given output signal; one band-pass filter, connected to the mixer, for passing a frequency band in which the side band signal appears, a frequency band in which the side band signal appears with respect to the main signal, and a frequency band in a turning position; and a detector for detecting the side band signal from an output of the band-pass filter.
When these means are taken, the side band signal appears on either one of an upper side and a lower side with respect to the main signal in the output from the orthogonal modulator. Moreover, when the phases of the I and Q signals generated from the base band signal are mutually reversed by the phase reverse means, an appearance position of the side band signal alternately changes to the lower side from the upper side or to the upper side from the lower side. This depends on a principle property of an orthogonal modulation system.
When the side band signal detector detects the side band generated in this manner, a distortion compensation action can be realized in the feedforward type linearizer. However, in the present invention, the appearance position of the side band signal is changed by changing the phase of the base band signal. Therefore, a changeover speed can be raised, and it is therefore possible to raise a control speed. Moreover, since only one band-pass filter and one detector may be disposed in the side band signal detector, size and cost can be reduced.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.